This invention is to a system and method for reducing methanol decomposition byproducts in a methanol to olefin reactor system. More particularly, the invention concerns reducing the formation of metal catalyzed side reaction byproducts by forming and/or coating at least a portion of the feed vaporization and introduction system, e.g., the feed introduction nozzle, with a material that reduces the formation of metal-catalyzed side reaction byproducts.
Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous important chemicals and polymers. Light olefins traditionally are produced by cracking petroleum feeds. Because of the limited supply and escalating cost of petroleum feeds, the cost of producing olefins from petroleum sources has increased steadily. Efforts to develop and improve olefin production technologies, particularly light olefins production technologies, have increased.
In an oxygenate to olefin (OTO) reaction system, a feedstock containing an oxygenate is vaporized and introduced into a reactor. Exemplary oxygenates include alcohols such as methanol and ethanol, dimethyl ether, methyl ethyl ether, methyl formate, and dimethyl carbonate. In a methanol to olefin (MTO) reaction system, the oxygenate-containing feedstock includes methanol. In the reactor, the methanol contacts a catalyst under conditions effective to create desirable light olefins. Typically, molecular sieve catalysts have been used to convert oxygenate compounds to olefins. Silicoaluminophosphate (SAPO) molecular sieve catalysts are particularly desirable in such conversion processes because they are highly selective in the formation of ethylene and propylene.
In a typical MTO reactor system, undesirable byproducts may be fanned through side reactions. For example, the metals in conventional reactor walls may act as catalysts in one or more side reactions. If the methanol contacts the metal reactor wall at sufficient temperature and pressure, the methanol may be converted to undesirable methane and/or other byproducts. Byproduct formation in an MTO reactor is undesirable for several reasons. First, increased investment is required to separate and recover the byproducts from the desired light olefins. Additionally, as more byproducts are formed, less light olefins are synthesized. In other words, the production of byproducts is undesirable because methanol feed is consumed to produce the byproducts. Further, although the relative concentrations of metal catalyzed side reaction byproducts are generally quite low, the total amount of byproducts produced on an industrial scale can be enormous. Thus, it is desirable to decrease or eliminate the synthesis of byproducts in an MTO reactor system.
Sulfur-containing chemicals have proven effective for deactivating or passivating the metal surface of a reactor thereby reducing the formation of undesirable byproducts in the reactor. For example, Japanese Laid Open Patent Application JP 01090136 to Yoshinari et al. is directed to a method for preventing decomposition of methanol or dimethyl ether and coking by sulfiding the metal surface of a reactor. More particularly, the method includes reacting methanol and/or dimethyl ether in the presence of a catalyst at above 450xc2x0 C. in a tubular reactor made of Iron and/or Nickel or stainless steel. The inside wall of the reactor is sulfided with a compound such as carbon disulfide, hydrogen disulfide or dimethyl sulfide. Additionally, a sulphur compound may be added to the feed.
Although passivating chemicals have proven effective in reducing metal catalyzed side reactions, the introduction of deactivating or passivating chemicals are problematic because these chemicals or their reaction products must be separated from the desired product. Thus, a need exists for a method and system for reducing the formation of metal catalyzed side reaction byproducts in an MTO reactor system while minimizing or eliminating the use of deactivating or passivating chemicals.
The present invention provides the ability to produce light olefins while reducing or eliminating the formation of metal catalyzed side reaction byproducts in a feed vaporization and introduction (xe2x80x9cFVIxe2x80x9d) system. An FVI system is the region of the reactor system beginning at the point that at least a portion of the feedstock is in a vaporized state and extending to the point that the feedstock exits the feed introduction nozzle and enters the MTO reactor. As the resulting light olefin stream contains less metal catalyzed side reaction byproducts than is produced in conventional MTO reactor systems, olefin separation and purification costs can be reduced. The resulting purified olefin stream is particularly suitable for use as a feed in the manufacture of polyolefins.
One embodiment of the present invention provides a method for forming light-olefins from an oxygenate-containing feedstock, including directing the feedstock through a feed introduction nozzle attached to an MTO reactor and having an inner surface, at least a portion of which is fanned of a first material resistant to the formation of metal catalyzed side reaction byproducts. As defined herein, a material that is xe2x80x9cresistant to the formation of metal catalyzed side reaction byproductsxe2x80x9d is less catalytically active to the formation of metal catalyzed side reaction byproducts than carbon steel. After entering the reactor volume, methanol in the feedstock contacts a catalyst under conditions effective to form an effluent comprising light olefins.
The present invention also provides a feed vaporization and introduction system for an MTO reactor, comprising a teed introduction nozzle including a first generally tubular member having a first end for receiving a feedstock from a heating unit, a second end adjacent a reactor unit, and an inner surface forming a conduit for delivering the feedstock from the first end to the second end. At least a portion of the inner surface is formed of a first material that is resistant to the formation of metal catalyzed side reaction byproducts.
Optionally, the temperature of the feedstock and/or at least a portion of the FVI system is controlled with a thermally insulating material or a cooling system to further reduce the amount of metal catalyzed side reaction byproducts that is produced.